This is an excerpt from the latest version perlfaq4.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .

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4.27: How can I access or change N characters of a string?

You can access the first characters of a string with substr(). To get
the first character, for example, start at position 0 and grab the
string of length 1.

$string = "Just another Perl Hacker";
$first_char = substr( $string, 0, 1 ); # 'J'

To change part of a string, you can use the optional fourth argument
which is the replacement string.

substr( $string, 13, 4, "Perl 5.8.0" );

You can also use substr() as an lvalue.

substr( $string, 13, 4 ) = "Perl 5.8.0";

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